Linux top shows several CPU measurements because they answer different questions: TIME/TIME+ are a process’s accumulated CPU time, the task-list %CPU is an interval reading, and the summary percentages describe CPU states across the system. They are not competing versions of one number.
Which CPU reading answers which question?
| Reading | Scope | What it measures | Time window |
|---|---|---|---|
TIME / TIME+ |
One task | Accumulated CPU duration | Since the task started |
Task-list %CPU |
One task | Task’s share of elapsed CPU time | Since the previous screen update |
| Summary CPU-state percentages | Whole system, or a CPU when per-CPU display is enabled | How CPU time was divided among states | Refresh interval |
ps %CPU |
One process | CPU time relative to elapsed time | Process lifetime |
The procps-ng top(1) manual, dated 2025-03-13, defines task-list %CPU as the task’s share of elapsed CPU time since the last screen update. The refresh interval is not a universal fixed number; it depends on the running top configuration.
What do TIME and TIME+ mean?
TIME is the total CPU time a task has accumulated since it started, not the wall-clock time it has been alive. TIME+ shows that same total with hundredths-of-a-second display precision. These fields measure duration, not a percentage or a recent sample.
A process can remain alive for hours while consuming little CPU, so its accumulated CPU time can be far shorter than its elapsed lifetime. A multithreaded process can also accumulate CPU time faster than wall-clock time when its threads execute simultaneously on multiple CPUs.
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Press S in top to toggle cumulative-time mode. When enabled, the listing includes CPU time consumed by dead child processes as well as the task’s own time.
Why can a process show more than 100% CPU?
In a true SMP environment, a multithreaded process can use more than one processor at once. In Irix mode, task %CPU can therefore exceed 100% when the process’s threads run concurrently on multiple CPUs.
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With Irix mode off, top uses Solaris mode and divides task CPU usage by the total number of CPUs. Press I to toggle between these modes. A percentage is only interpretable alongside the mode, CPU count, and whether the display is listing tasks or individual threads.
What do the CPU summary percentages mean?
The summary line divides CPU time during the refresh interval into states. The labels in the default summary are:
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us: time spent running unniced user processes.sy: time spent running kernel processes.ni: time spent running niced user processes.id: time spent in the idle handler.wa: time waiting for I/O completion.hi: time servicing hardware interrupts.si: time servicing software interrupts.st: time stolen from a virtual machine by its hypervisor, when the field is available.
The Linux time(7) manual distinguishes user and system CPU time by where execution occurs: user time is spent executing code in user mode; system time is kernel execution on a process’s behalf, such as while handling a system call. The st field may be absent depending on kernel version.
Why do top and ps show different CPU percentages?
The two tools use different time windows. In procps, top task %CPU reflects the interval since the previous screen update. The ps(1) manual defines %CPU as CPU time divided by elapsed time over the process lifetime. A process that has recently become busy or idle can consequently have different readings in the two commands.
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How does Linux account for CPU time?
Linux exposes system and per-CPU counters in /proc/stat. Its fields include user, nice, system, idle, iowait, irq, softirq, and steal. These counters are measured in USER_HZ units, not inherently in seconds. The proc_stat(5) manual says this is 1/100th of a second on most architectures and recommends using sysconf(_SC_CLK_TCK) to obtain the value for a system.
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A quick way to diagnose a mismatch
- Identify the field. Decide whether you are comparing accumulated
TIME+, task%CPU, a summary state, orps%CPU. - Match the time window. Separate since-start totals, since-refresh readings, and lifetime averages.
- Check normalization. Note whether Irix or Solaris mode is active and how many CPUs the machine has.
- Check what is listed. Determine whether
topis showing processes/tasks or threads, and whether cumulative-time mode is enabled. - Check the local manual. The article describes Linux procps-ng
top; other Unix-like systems can use different implementations. For a particular host or screenshot, consult that system’sman top. Visible fields can also vary by kernel version.
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